Intrusion detectors operating within the UHF region of radio transmission are known for the detection of the movement of objects (preferably human) within an area of coverage. However, all such designs possess inherent causes of instability. A fundamental redesign of the method by which such detectors operate, has resulted in this invention.
Previously known detectors have incorporated an oscillator circuit comprised of one or move valves or transistors. This oscillator is coupled to an antenna. This antenna is used both for transmission and reception of the radio signal derived from the oscillator. The transmitter signal fills the area of coverage and is reflected back off each object within the area. This reflected signal is received on the antenna. Due to the phenomenon of the Doppler effect, if an object within the area is moving, then the frequency of the reflected and received signal will be different from the frequency which was transmitted. These transmitted and received signals are mixed at the antenna to produce the Doppler difference frequency; corresponding to the difference between the transmitted and received frequencies.
The usual method for obtaining this Doppler difference frequency is to use a mixing detector diode. This diode connects to a pickup loop coil. This coil is connected at one end to the antenna and at the other end to chassis earth, while the diode connects to a centre tap. In some configurations, two coils and two diodes are used in a balanced configuration. Other configurations have been tried which employ the diode connected to the oscillator tank coil.
There are three major limitations of this design. Firstly, variations in the quality of the diode from product to product cause changes in the range of area coverage attainable, causing problems in production quality control. Secondly, for satisfactory operation of the diode, relatively high transmitter power is required, which can in turn give rise to interference to other receivers operating on the same frequency (including other UHF movement detectors). Thirdly, the use of a receiver makes the detector vulnerable to the reception of interfering radio signals from any other type of transmitter (e.g. taxi radio).
Furthermore, this receiver is of very broad band reception which exacerbates the likelihood of receiving interference. This broadness of bandwidth is caused by the low Q of the antenna loop or tank coil, and cannot be improved without relatively complex double or triple conversion techniques.
Other aspects requiring improvement relate to the amplification and processing of the signal derived from the transmitter.